1. Technical Field
This invention relates to batteries, and in particular, batteries based on lead chemistry.
2. Art Background
Batteries, such as lead acid batteries, are ubiquitous in today's society. Lead batteries are employed for many Uses such as in the electrical systems of automobiles. Additionally, these batteries are employed in applications such as emergency power for telecommunication systems where occasional, but extremely reliable, use is necessitated. Such diversified use imposes an equal diversity of required properties.
The structure of the positive plate of a lead based battery is a primary factor affecting its life and its current generating efficiency. Lead dioxide is employed as the active positive material. Typically, a paste of a precursor to the lead dioxide is applied to a lead grid to make the positive plate. The precursor is then electrochemically oxidized to the lead dioxide.
In particular, for conventional positive plate fabrication, powders of Pb, PbO, Pb.sub.3 O.sub.4 or their mixtures are mixed with water and H.sub.2 SO.sub.4 to form a paste with good adhesivity to the lead grid. This paste, depending on the ratio of starting materials, rate of mixing and the temperature, contains mixtures of the initial powders, lead sulfate, and basic lead sulfates such as PbOPbSO.sub.4, 3PbOPbSO.sub.4.H.sub.2 O, and 4PbOPbSO.sub.4 (TTB). The paste is applied to a lead or lead alloy grid and the plates are cured. Curing consists of exposing the plates to a controlled environment of temperature and humidity, where further reaction of the ingredients occur, resulting in a different ratio of the lead oxides, sulfate, and the basic lead sulfates. The cured plates are then immersed in sulfuric acid where, in a step denominated formation, the paste material is electrochemically oxidized to PbO.sub.2, the active material of the positive plate of the lead acid battery.
TTB, which crystallizes as large elongated prismatic (needle shape) crystals, undergoes anodic conversion to PbO.sub.2 without losing the prismatic structure. The interlocking of these prismatic crystals provides mechanical strength to the positive plate and are thus less susceptible to shedding during battery cycling. For this reason, batteries for deep cycling are generally manufactured with a large amount of TTB in the active material at the end of the curing process. The large crystals of TTB typically employed provide strength to the positive electrode during use, but their formation is inefficient and their utilization (capacity per gram of active material) is lower than other oxides. Indeed, electrodes made with a large amount of TTB have up to 25% less capacity than conventional electrodes and often require up to 30 deep charge-discharge cycles to reach their rated capacity.
Biagetti (U.S. Pat. No. 3,765,943, dated Oct. 16, 1973) discloses a process to produce TTB crystals by reacting, at a temperature above 70.degree. C., stoichiometric ratios of orthorhombic PbO and sulfuric acid in an aqueous solution using a slowly stirred reaction vessel. Plates are made by mixing the resulting TTB with water and applying the resulting paste to a lead grid. These plates containing essentially 100% TTB have long life. However, as discussed, the oxidation of the TTB to PbO.sub.2 is relatively inefficient due to the large size of the crystals employed.
Thus, although prismatic crystals of TTB improve the adhesion of active material during use, their performance has not been entirely satisfactory. The capacity per gram of active material is generally lower than plates formed by conventional techniques. Additionally, conversion from the precursor into the active material is relatively slow. Improvement in conventional lead acid batteries, and in particular, the lifetime of such batteries, is quite desirable. Approaches for use of new precursor crystals have indeed generated hope that lifetime can be improved. Nevertheless, these approaches, although offering enhanced lifetimes, often yield batteries that are somewhat inefficient to produce and that have reduced capacity per gram of active material.